GB2098400A - Remote mains switching system - Google Patents

Abstract

A remote mains switching system has socket outlets switched from a central location by signals that are transmitted to the socket outlets along conductors of the mains supply. Switching of the supply at each socket outlet is effected by means of a self- holding relay 7. A mild steel core (11) has mounted thereon a coil (10) including two windings (L1, L2) wound in opposite senses. The core is mounted on a yoke (12) which cooperates with a hinged armature (13) carrying switching contacts (MS1, 2) for the mains supply. The windings (L1, L2) are connected to a discharge capacitor (C1) through respective switching transistors (TR1, TR2). Upon switching on of transistor TR1, a current pulse produced in winding L1 produces a transient magnetic field that attracts the armature (13) to the core (11) and also produces a remanent magnetic field in the core (11) to retain the armature. Upon operation of the transistor TR2 an opposite transient magnetic field is similarly produced by winding L2 thereby destroying the remanent field and allowing a spring (17) to pull the armature away from the core. <IMAGE>

Description

SPECIFICATION Remote mains switching system This invention relates to a remote mains switching system wherein remote mains outlets are switched by signals produced at at least one central transmitting station and sent to the remote outlets, for the purpose of controlling the supply of electrical power from the outlets to appliances, lighting installations and like electrical power consuming devices.

Such a system is in general terms known, for example in "The Intelligent Plug" Wireless World December 1979, wherein there is described a system in which FSK command signals are transmitted along conductors of a mains a.c.

supply, to control the switching state of various outlets connected to the supply. the outlets are addressed individually by providing individual address codes for the transmitted command signals. Each outlet includes a receiver circuit which in response to the correct address code and the received command, switches the conductivity state of the outlet. The switching of electrical power at the outlet is performed by a triac.

However, for mains power switching, the inherent characteristics of a triac involve some serious drawbacks. Devices such as triacs and bipoiar transistors, in which the principle current flows across a P-N junction, place a constant voltage drop in series with the principle current.

For a triac, the resulting power dissipation in the device is approximately 1W per Amp of principle current.

Under conditions of current overload, the sudden increase in power dissipation, in the triac, may cause it to fail in preference to the circuit fuse. In order to avoid this, it is necessary to choose a triac with the correct thermal characteristics to withstand the 12t rating of the protection fuse. Inevitably the triac chosen will be determined by its overload characteristics, and for a triac rated at 13 amp and protected by a 1 3A fuse, the cost of the triac becomes unduly expensive.

Similarly, voltage rating in the off state is an important consideration with triacs. The short current path, which in the ON state leads to lower power dissipation, in the OFF state leads to a higher electric field in the semiconductor material and heightens the chance of avalanche breakdown.

Since a triac does not break the circuit by a mechanical contact, there is a small leakage current (--.1 mA) when the switch is off. In applications where safety isolation must be provided, semiconductors may not be used without an additional mechanical circuit breaker.

In contrast, the present invention utilises a self latching relay system to switch mains power at the socket outlet and an object of the present invention is to provide a relay which is sufficiently compact, cost effective and reliable for switching a.c. mains current in a remote mains switching system.

Self latching relays are known per se. It is known to provide a self latching relay with two coils wound on separate bobbins which move respective armatures, the armatures being connected as a common assembly which is moved back and forth between a latched and a released position by respective energisations of the coils, the assembly operating switching contacts. However, such a relay cannot be made sufficiently small to fit conveniently within a mains socket outlet and provide a practical performance for a remote mains switching system.

Another known self latching relay comprises a single coil which drives a spring loaded armature, the armature operating a mechanical linkage including a ratchet mechanism which operates switching contacts, the arrangement being such that successive energisations of the coil cause the switching contacts to be successively opened and closed. However, the complexity of the mechanical linkage makes such a relay relatively bulky and not entirely suited to a remote mains switching system.

A further known form of self latching relay comprises a single coil wound around a core that includes a permanent magnet. A magnetically permeable metal armature is pivotally mounted for moving to a latched position in engagement with the magnetic core, where it is held in position by the magnet, and a released position away from the core. A spring pulls the armature to the released position. The winding needs to be energised to move the armature to the latched position whereupon it is held by the magnet.

Subsequent energisation of the coil with a reversed field releases the armature.

In the system of the present invention the self latching relay includes a coil, a movable armature and a yoke. The armature moves between a latched and a released position, and operates switching contacts for switching the mains supply. A capacitor is arranged to discharge through the coil so as to create a magnetic field to move the armature and complete a magnetic circuit which remains completed by magnetic remanance produced by the capacitor discharge.

The armature is released by producing a further capacitor discharge through the coil in such a manner as to destroy the remanant field and thereby release the armature.

The relay has the advantage that due to its pulsed operation the coil does not have to be fed with current continuously, and hence does not dissipate power and hence heat continuously.

Furthermore, the self latching action of the relay provides an inbuilt "memory" which is particularly useful where mains-borne spikes are present. In contrast, the conventional relays and triacs tend to change their switching state when mains supply interruptions of a few milliseconds, or mains spikes, occur.

The relay utilised in the present invention, compared with the prior self latching relays aforesaid offers advantages of simplicity, compactness and cost effectiveness for use in a remote mains switching system.

Also, the relay used in the system of the present invention does not generate ratio frequency interference, as with triacs.

With the relay used in the remote mains switching system of the invention, it is possible by selection of the material of the switching contacts and by the provision of a relatively high contact pressure (e.g. 50 gm) to render the contacts able to withstand the l2t rating of a 81362 protection fuse without contact welding. Also, the relay contacts when unlatched provide a minimum gap e.g. 1 mm for isolation purposes, avoiding the need for an additional circuit breaker.

In an embodiment of the invention to be described in more detail hereinafter, auxilliary switching contacts are provided, operating at a low voltage, of say 1 5 volts to provide information as to the latched or released state of the relay.

This information is fed back through the system to provide an indication that a command signal has been carried out.

The described embodiment of the relay provides a device suitable for switching currents of up to 16 amps, thus allowing drive circuitry for the relay to be standardised for the range of mains current switching operations usually encountered in a domestic a.c. mains supply.

In order that the invention may be more fully understood an embodiment thereof will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a block circuit of a socket outlet for a single phase domestic a.c. mains supply, which incorporates a relay for switching the socket outlet, the relay being controlled by signals transmitted along the mains supply conductors from a remote location.

Figure 2 is a perspective view of the relay of Figure 1, showing schematically the layout of its major component parts, Figure 3 is a detailed side view of the relay, Figure 4 is a view of the mounting block 22 shown in Figure 3, with the relay coil, yoke and movable contacts removed, Figure 5 is a sectional view of the relay taken along the line A--A' of Figure 4, Figure 6 is a bottom plan view of the relay, Figure 7 is a top plan view of the relay, Figure 8 is a perspective view of the insulating support block for the movable contacts, showing movable contact MS1, and Figure 9 illustrates steering logic for use in the circuit 8, to enable control of the relay by a local override switch.

Referring firstly to Figure 1, there is shown schematically a socket outlet for a domestic a.c.

mains supply. The supply includes live and neutral conductors 1, 2 operating at mains a.c. voltage, for example 240 volts. An earth conductor 3 is also provided. The conductors terminate at sockets 4, 5, 6 which receive pins of a plug (not shown) for connection to an electrical appliance.

The socket includes a relay shown schematically within hatched outline 7, the relay comprising first and second coil windings L1, L2, a set of switching contacts MS 1, 2 connected in the live conductor 1, and a set of changeoever contacts SS 1 to 3 for switching a low voltage supply so as to signal the operating condition of the relay.

As is explained in more detail hereinafter, the winding L1 is for opening the contacts MS1, 2 and the winding L2 is for closing the contacts, the opening or closing of the contacts MS 1, 2 being accompanied by a changeover of the contact SS2 from engagement with SS 1 to SS3 or vice versa.

The relay 7 is operated in response to command signals transmitted from a central controller (not shown) at a remote location, the command signals being transmitted along the conductors 1, 2. The command signals are received by a transmitter/receiver circuit 8 which in response to a command to open the relay contacts MS1, 2, produces a pulse on line 8A to momentarily switch on a normally off switching transistor TR 1 connected in series with the winding L1. When the received command is to close the contacts MS 1, 2, the circuit 8 produces a pulse on line SB to momentarily switch on a transistor TR2 connected in series with the winding L2.

The switching state of the relay can also be controlled by a local override switch which overrides the commands received by the circuit 8.

The local override switch will be described in more detail hereinafter with reference to Figure 9.

A low voltage power supply 9 is connected to the live and neutral conductors 1, 2 and produces a low voltage d.c. supply of typically 15 volts which is applied through a resistor R1 to charge a capacitor C1 connected in parallel with each of the windings L1, L2. The 1 5 volt supply is also connected across the switching contacts SS1, SS3 in order that the contact SS2 assumes a potential of O or 1 5 volts depending on whether it contacts SS1 or SS3. The potential of SS2 thus is a signal indicative of the switching state of the relay, and this signal is fed back on line 8C to the transmitter/receiver circuit 8.

In use, when the central controller commands the socket outlet to switch off, an appropriate command signal is transmitted along the conductors 1, 2 and is detected by the transmitter/receiver circuit 8. TransistorTR1 is consequently switched on momentarily, which causes capacitor C1 to discharge through the winding L1 to activate the relay 7 to open contacts MS 1, 2, and to changeover SS2 into contact with SS 1. Contact SS2 thus applies a O volt potential back to transmitter/receiver circuit 8, which transmits an appropriate signal back through the conductors 1, 2 to the central controller, to indicate that the transmitted command has been carried out by the relay 7.

Similarly, when the central controller commands the socket outlet to switch on power, the transmitted command signal is coded to cause the circuit 8 to switch on transistor TR2 momentarily, causing capacitor C1 to discharge through winding L2, and causing the relay contacts MS1, 2 to close. Contact SS2 changes over its position to contact SS3, which applies a 1 5 volt signal to circuit 8 to cause an appropriate response signal to be transmitted back to the central controller to indicate completion of the command.

The physical arrangement of the major components of the relay 7 will now be described with reference to Figure 2.

The windings L1, L2 form a coil 10 and are concentrically wound in opposite senses about a solid cylindrical metallic core 11 disposed within a generally U-shaped metallic yoke 12. The yoke 12 comprises an armature 13 hingedly mounted on a stator part 14, the core 11 being fixedly mounted on the part 14. Both the stator part 14 and the armature 13 are provided with rearwardly extending arms 15, 1 6 to which are attached a tension spring 1 7.

The hinge connection between the armature 13 and the stator 14 is defined by upstanding lugs 1 8 in the stator which fit into cooperating recesses 1 9 in longitudinal side edges of the stator. The spring 1 7 acts in a manner to hinge the major portion of the armature in a direction to move out of contact with the core 11. The movable contacts MS1 and SS2 are mounted for movement with the armature 13, and the movable contacts comprise elongate resilient metallic fingers clamped in an insulating support block 20 mounted on the armature.

The other contacts are mounted in fixed positions relative to the stator 14, in the positions shown.

When the coil winding L1 is energised as described with reference to Figure 1, a magnetic field is set up in the yoke 12 by the coil 10, and causes the armature 1 3 to be attracted into engagement with the core 11, the armature pivoting at the hinge 18, 1 9 against the tension force of the spring 17. The arrangement of the armature 13 and the contacts is such that the contact MS1 engages contact MS2 before the armature 1 3 moves into engagement with the core 1 Thus, as the armature moves downward to engage the core 1 the contact MS 1 bends resiliently and consequently establishes a contact pressure between the contacts MS1, MS2.

When the current pulse through the winding L1 ceases, the armature 13 remains engaged with the core 11 by magnetic remanance.

Thus the effect of the transitory magnetic field produced in the winding L1 is to produce a remanent field in the yoke 12 and the core 11, the remanent field being of sufficient strength to hold the armature 1 3 in contact with the core 11.

The materials of the yoke 12 and the core 11 are selected to be of a high susceptibility and coercivity so as to have a high retentivity with retentivity with regard to the applied field. The yoke 12 and the core 11 can conveniently be made of low grade mild steel, in contrast to the more expensive materials used in conventional relays. Also, the relay is arranged to minimise flux leakage from the yoke 12, as will be explained in more detail hereinafter.

By providing sufficient ampere turns for the winding L1, it is possible to give the relay a measure of mechanical shock resistance. When the relay is latched, if the remanant field exerts for example a force at 200 grams at the armature 13, the contact pressure is 50 grams, and the pull-off force of the spring 1 7 is 80 grams, a surplus force of 200-(80+50)=70 grams acts on the armature 13 this surplus force providing a vibration resistance for the armature.

When the winding L2 is subsequently energised, it establishes a transitory magnetic field of opposite sense to the remanant field and of sufficient strength to release the armature 13, such that the spring 17 can pull the armature upwardly and open a gap between the contacts MS1, 2. It will be appreciated that the field strength required to release the relay is substantially less than that required to latch it, and hence the winding L2 requires fewer turns than winding L1 or can utilise reduced diameter wire.

A detailed description of a practical form of the relay will now be given with reference to Figures 3 to 8. Referring to Figure 3, the relay is shown mounted on a printed circuit board 21 and the various connections for the contacts and the coil windings are made direct to the board, as will be explained hereinafter.

The relay includes an insulated mounting block 22 moulded in nylon, to which the yoke 12 is attached by a screw 23. The yoke 12 is made of mild steel sheet. The stator part 14 is made by stamping such that an integral part is bent outwardly to define the attachment 1 6 for the spring 17.

The coil 10 is wound on a nylon bobbin 24.

The core 11 comprises a solid cylindrical shank having a head portion 11 a to engage the armature and a reduced diameter tail portion 11 b which fits through an aperture 25 in the stator 14 and which is deformed into an outwardly extending flange so as to hold the core 11 and the coil 10 in position. The flanging of the core portion 11 h ensures intimate contact at the joint between the core 11 and the stator part 14 so as to reduce magnetic flux leakage.

The form of the hinge between the armature 13 and the stator part 14 can be seen in Figure 7.

The arrangement of the lugs 18 and the recesses 1 9 is such that a substantial contact area is provided between the stator 14 and the armature 13, and moreover, the spring 1 7 establishes a contact pressure at the hinge to produce intimate contact. The combination of these factors minimises flux leakage from the hinge, so as to maximise the remanant field established when coil winding L1 is energised.

The insulating support block 20, which holds the movable contacts M81 and SS2, is held on the armature 13 by two tubular brass rivets 26 shown in Figure 7. A schematic perspective view of the support block 20 is shown in Figure 8 with the contact MS1 being shown mounted in position, but with the contact SS2 removed. The block 20 is moulded in nylon and has a central upstanding dividing wall 27 to isolate the high and low voltage contacts MS1,SS2. The contact MS 1 is fitted into the block 20 by sliding the contact transversely of its length between an upper support block 28 and lower support lands 29, 30. The contact MS1 is held in position by a re-entrant integral flange 31 which acts like a barb.The contact is prevented from longitudinal siding motion by virtue of it having a generally Lshaped end portion 32 entrapped between a bulbous extension 33 of the support block 28 and a transverse back flange 34.

The contact SS2 is similarly located beneath a support block 35.

The block 20 is also provided with a flange 36 the purpose of which can be best seen from Figure 3. As previously mentioned, when the relay latches and the armature 1 3 engages the core 11, the contact MS1 flexes to establish a contact pressure between MS 1 and MS2. The contact MS 1 is shown in such a flexed condition in Figure 3. When the relay releases, the armature 13, the block 20 and contact MS1 move to the position shown in hatched outline. As the armature 13 pivots upwardly, the flange 36 moves up into engagement with the contact MS 1 so as to positively lever it upwardly of the fixed contact MS2, thereby overcoming any adherence between the contacts which might otherwise cause the relay to stick in its latched position.

As is shown in Figures 3, 7 and 8, the support block 20 has a stop surface 37 which cooperates with a projection 38 on the mounting block 22 to limit upward movement of the armature.

The electrical circuits from the printed circuit board 21 through the contacts will now be described.

The live lead 1 of Figure 1 is connected to a copper contact 39 (Figure 3), to which is soldered a steel bridging conductor 40, which -makes electrical connection with the end portion 32 of the mains contact MS1. Contact MS1 is conveniently made from phosphor bronze strip and is fitted with a silver cadmium oxide contact head 41. The contact MS2 is conveniently made of copper and fitted with a silver cadmium oxide contact head 42. The contact MS2 is connected to the terminal 4 of Figure 1.

Referring to Figure 5, the 1 5 volt supply from the circuit 9 of Figure 1, is applied to the changeover contact SS2 through a contact plate 43 and a bridging conductor 44. The contact SS2 is conveniently made from a phosphor bronze strip and has a double sided contact head 45 for engaging cooperating contact heads 46, 47 on the fixed changeover contacts SS1, SS2.

Referring now to Figure 4, connections to the windings L1, L2 are brought back to the printed circuit board by means of contact pins 48, 49, 50 mounted in the mounting block 20.

Reference will now be made to Figure 9, which iilustrates the local override switch, and steering logic for operating the relay in response to operation of the override switch.

The local override switch comprises a pushbutton switch SW which upon depression earths a control line 51. Thus a zero level on line 51 indicates an override signal for changing the switching state of the relay. The switching lines 8A, 88 of Figure 1 are shown in Figure 9, the lines being for switching the transistors TR 1 and TR2 which respectively latch and release the relay. A positive voltage or "1" signal on line 8A will thus latch the relay, whereas a "1" signal on line 8B will release the relay.

D-type flip-flops 52, 53 receive at their D inputs data received by the circuit 8 (Figure 1) regarding commands transmitted from the central station along the mains supply. A "1" applied to the D input of flip-flop 52 is for latching the relay whereas a "1 " applied to the D input of flip-flop 53 is for releasing the relay.

The line 8C receives a voltage from changeover contacts SS 1--3 (Figure 1). A positive voltage on line 8C represents a "1" which indicates that the relay is latched; a "0" indicates that the relay is released. Steering logic comprising NAND gates 54 to 59, OR gates 60, 61 and an inverter 62 is so arranged that when the override pushbutton SW is operated, the latched or released state of the relay as determined by the states of the flipflops 52, 53, is overridden and the relay is caused to change state.

When the override switch SW is operated, it applies a "0" input to each of the OR gates 60, 61. The other inputs of the OR gates 60, 61 are connected to the outputs of NAND gates 56, 57 which are connected to define a bistable. The bistable applies a "1" and a "0" input to gates 60, 61 or vice versa depending on whether the bistable is set or reset. NAND gates 54, 55 and inverter 62 control the setting and resetting of the bistable 56, 57.

To illustrate the operation of the logic, the situation will be considered in which the circuit 8 has received a signal, transmitted along the mains supply, which latches the relay. Thus, the circuit 8 applies a "1" pulse to the D input of flip-flop 52, which produces a "1" signal on line 8A, through gate 57, and latches the relay. Consequently, the changeover contacts produce a "1" signal on line 8C. A "1" signal initially occurs on the local override line 51. Thus, NAND gates 54 and 55 set the bistable gate arrangement 56, 57 so that gate 56 applies a "1" to OR gate 60 and gate 57 applies a "0" input to OR gate 61. Consequently gate 61 produces a "1" output. Gate 58, which controls the resetting of the relay therefore produces a "0" output since it receives a "1" input from Q of flip4lop 53 and a "1" from gate 61. However, when the switch SW is operated gate 61 produces a "0" output pulse which causes the output of gate 58 to go high and reset the relay, thus overriding the latching command received by the circuit 8.

During this operation gate 57 has received a "1" input from OR gate 60 and the Q output from flip-flop 52. This Q output is normally "1" but is "0" when a "1" latching pulse is applied by the circuit 8 to the D input.

Thus the gate 57 produced a "1" output pulse in response to the "1" pulse applied to the D input of flip-flop 52. The logic circuit has a feature which prevents the line 8A spuriously becoming permanently locked into a "1" state. If for some reason the flip-flop 52 becomes set in a condition where Q remains at "0" in response to a "1" applied to its D input when the override switch SW is operated, the consequential "0" output from gate 61 is applied to the clear input of flipflop 52 and will clear it. The first operation of the pushbutton SW will not switch the relay in this situation because the capacitor C1 (Figure 1) will have been discharged by the continuous "1" signal on line 8C. However, a second operation of pushbutton SW will switch the relay.

When the pushbutton SW is operated to release the relay, the signal on line 8C from the changeover contacts will to to "0", and the bistable gate arrangement 56, 57 changes state, so that the local override signal on line 51 is steered through OR gate 60 to operate NAND gate 57 to latch the relay.

The effect of successive operations of the pushbutton SW therefore is to toggle the relay on and off.

Typically the local override switch SW will be a pushbutton switch mounted on or in association with the socket outlet, but it will be appreciated that the switch SW could be an electronic switch itself controlled or situated at a remote location.

Thus, there is provided a compact relay suitable for use in a socket outlet for controlling switching of mains power to an appliance, under the control of signals sent from a remote location.

In a practical example of a remote mains system, there will be provided a plurality of socket outlets similar to that described, each of which can be addressed separately by individually coded command signals produced at the central location.

Also, instead of transmitting the command signals along the mains a.c. conductors, the signals could be transmitted by other communication techniques in order to reach the socket outlets. Furthermore, it will be appreciated that the described system can be used to switch mains outlets other than socket outlets, for example ceiling roses for lighting installations.

Claims (Filed on 13 May 1982) 1. A self latching relay comprising switching contacts, a magnetically permeable member, an armature mounted for movement between a first position relatively close to said member and second position relatively remote from said member, spring means urging the armature to the second position, said movement of the armature producing switching of said contacts, coil means for inducing a magnetic field in said magnetically permeable member, means for producing a transient current flow through the coil means in such a manner as to produce a transient magnetic field which results in attraction of the armature to the first position said magnetic field also inducing in the magnetically permeable member a remanent field which maintains said armature in the first position against the force of the spring means and after the occurence of said transient magnetic field, and means for producing a transient current flow through the coil means in such a manner as to produce a transient magnetic field which destroys said remanent field sufficiently for the spring means to move the armature to the second position.

2. A relay according to claim 1 wherein said magnetically permeable member comprises an elongate core passing through the coil means and mounted at one end on a yoke of magnetically permeable material, the yoke comprising a stator and the armature which is hinged on the stator, the armature in said first position abutting the other end of the core.

3. A relay according to claim 2 wherein said core and said yoke are made of mild steel.

4. A relay according to any preceding claim wherein said coil means includes first and second windings; and said transient current flow producing means includes charge storage means, means for discharging the charge storage means through the first winding to produce said field which attracts the armature to the first position and produces said remanent field, and means for discharging the charge storage means through the second winding to produce said destruction of the remanent field.

5. A relay according to claim 4 wherein said charge storage means comprises a capacitor connected mutually in parallel with said coil windings and said discharging means includes first and second switching transistors connected in series with said windings respectively.

6. A relay according to any preceding claim wherein said switching contacts include a first contact mounted in a fixed position relative to said magnetically permeable member, and a second contact mounted on the armature and arranged to engage the first contact when the armature moves to said first position.

7. A relay according to claim 6 wherein said contacts are arranged to engage one another before said armature reaches said first position, and at least one of the contacts is resiliently deformable such that upon the armature reaching said first position a contact pressure is established between the contacts.

8. A relay according to any preceding claim wherein said switching contacts include a changeover contact mounted on the armature, and further contacts mounted in fixed positions relative to the magnetically permeable member, the changeover contact engaging different ones of the further contacts when the armature is in said first and second positions respectively.

9. A self latching relay substantially as

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (14)

**WARNING** start of CLMS field may overlap end of DESC **. "1" input from OR gate 60 and the Q output from flip-flop 52. This Q output is normally "1" but is "0" when a "1" latching pulse is applied by the circuit 8 to the D input. Thus the gate 57 produced a "1" output pulse in response to the "1" pulse applied to the D input of flip-flop 52. The logic circuit has a feature which prevents the line 8A spuriously becoming permanently locked into a "1" state. If for some reason the flip-flop 52 becomes set in a condition where Q remains at "0" in response to a "1" applied to its D input when the override switch SW is operated, the consequential "0" output from gate 61 is applied to the clear input of flipflop 52 and will clear it. The first operation of the pushbutton SW will not switch the relay in this situation because the capacitor C1 (Figure 1) will have been discharged by the continuous "1" signal on line 8C. However, a second operation of pushbutton SW will switch the relay. When the pushbutton SW is operated to release the relay, the signal on line 8C from the changeover contacts will to to "0", and the bistable gate arrangement 56, 57 changes state, so that the local override signal on line 51 is steered through OR gate 60 to operate NAND gate 57 to latch the relay. The effect of successive operations of the pushbutton SW therefore is to toggle the relay on and off. Typically the local override switch SW will be a pushbutton switch mounted on or in association with the socket outlet, but it will be appreciated that the switch SW could be an electronic switch itself controlled or situated at a remote location. Thus, there is provided a compact relay suitable for use in a socket outlet for controlling switching of mains power to an appliance, under the control of signals sent from a remote location. In a practical example of a remote mains system, there will be provided a plurality of socket outlets similar to that described, each of which can be addressed separately by individually coded command signals produced at the central location. Also, instead of transmitting the command signals along the mains a.c. conductors, the signals could be transmitted by other communication techniques in order to reach the socket outlets. Furthermore, it will be appreciated that the described system can be used to switch mains outlets other than socket outlets, for example ceiling roses for lighting installations. Claims (Filed on 13 May 1982)

1. A self latching relay comprising switching contacts, a magnetically permeable member, an armature mounted for movement between a first position relatively close to said member and second position relatively remote from said member, spring means urging the armature to the second position, said movement of the armature producing switching of said contacts, coil means for inducing a magnetic field in said magnetically permeable member, means for producing a transient current flow through the coil means in such a manner as to produce a transient magnetic field which results in attraction of the armature to the first position said magnetic field also inducing in the magnetically permeable member a remanent field which maintains said armature in the first position against the force of the spring means and after the occurence of said transient magnetic field, and means for producing a transient current flow through the coil means in such a manner as to produce a transient magnetic field which destroys said remanent field sufficiently for the spring means to move the armature to the second position.

2. A relay according to claim 1 wherein said magnetically permeable member comprises an elongate core passing through the coil means and mounted at one end on a yoke of magnetically permeable material, the yoke comprising a stator and the armature which is hinged on the stator, the armature in said first position abutting the other end of the core.

3. A relay according to claim 2 wherein said core and said yoke are made of mild steel.

4. A relay according to any preceding claim wherein said coil means includes first and second windings; and said transient current flow producing means includes charge storage means, means for discharging the charge storage means through the first winding to produce said field which attracts the armature to the first position and produces said remanent field, and means for discharging the charge storage means through the second winding to produce said destruction of the remanent field.

5. A relay according to claim 4 wherein said charge storage means comprises a capacitor connected mutually in parallel with said coil windings and said discharging means includes first and second switching transistors connected in series with said windings respectively.

6. A relay according to any preceding claim wherein said switching contacts include a first contact mounted in a fixed position relative to said magnetically permeable member, and a second contact mounted on the armature and arranged to engage the first contact when the armature moves to said first position.

7. A relay according to claim 6 wherein said contacts are arranged to engage one another before said armature reaches said first position, and at least one of the contacts is resiliently deformable such that upon the armature reaching said first position a contact pressure is established between the contacts.

8. A relay according to any preceding claim wherein said switching contacts include a changeover contact mounted on the armature, and further contacts mounted in fixed positions relative to the magnetically permeable member, the changeover contact engaging different ones of the further contacts when the armature is in said first and second positions respectively.

9. A self latching relay substantially as

herein before described with reference to the accompanying drawings.

10. A switched electrical outlet for use in a remote mains switching system, including electrical outlet conductors for the mains supply, a relay according to any preceding claim the switching contacts thereof are arranged to switch selectively mains supply through the conductors receiver means for receiving from a remote location switching commands for the relay, and means for producing said transient current flows through the coil means of the relay in dependance upon the switching commands received by the receiver means.

11. A switched electrical outlet according to claim 10 wherein said switching contacts include said changeover contact and said further contacts as defined in claim 8, and including means for supplying to the changeover contact an electrical reference potential derived from the mains supply, means responsive to which of said further contacts receives said reference potential from the changeover contact whereby to determine the switching state of the relay, and means for transmitting to said remote location a signal indicative of said switching state.

12. A switched electrical outlet according to claim 11 including local override means for overriding switching commands received by the receiver means, and an electrical logic circuit responsive to the switching state of the relay as defined said further and changeover contacts and to a local override command produced by said local override means, said logic circuit being arranged to control said transient current flows in the coil means to change the switching state of the relay in response to said local override command.

1 3. A switched electrical outlet for use in a remote mains switching system, substantially as hereinbefore described with reference to the accompanying drawings.

14. A remote mains switching system including a plurality of switched electrical outlets each according to any one of claims 10 to 13 and connected to a mains supply, and a central controller for transmitting to the outlets commands for switching the outlets selectively.

1 5. A system according to claim 14 wherein said central controller is arranged to transient said commands along conductors of the mains supply.

1 6. A remote mains switching system substantially as hereinbefore described with reference to the accompanying drawings.